Maybe try a dowel that matches the interior of the extrusion . . . either extruded directly over the dowel, or inserted after. I'd either put plastic or newspaper under the extruded forms so they can shrink without binding/deforming on a bare wareboard or put a smaller dowel through them and suspend them so they air dry without laying flat on a board. Minimum handling.

I work 99% in porcelain for some time now.If you are making only a few extrude a tube and leave it hang till firmer-Which will keep it straight and easier to handle,then hold a board up to its length for support as you cut it-a helper would be good. I would have paper on the board and lay some soft fabric on each side for support as it dries under plastic slowly.
If you are production making them then I would have some more steps for you.
I used to make production porcelain extruded glasses but straight was not important.
Mark